A conventional method for reducing loss of a surface acoustic wave (SAW) device is known in which electrode groups each consisting of an interdigital electrode and a common portion are spaced by n±¼ of a wavelength (where n is an arbitrary integer), and a phase shifter is connected to allow a surface acoustic wave to propagate in a single direction for reducing a conversion loss.
This method can effectively reduce the conversion loss by preventing bi-directional excitation of the surface acoustic wave, but increases a loss as a result of the bidirectional wave propagation at some portions since interdigitating portions functioning as excitation sources of the interdigital electrodes are not necessarily arranged on the same propagation path.
To solve this problem, a space between a finger of the interdigital electrode and the common portion adjacent to the finger is determined within ⅛ of the wavelength, and the finger of the interdigital electrode is provided with a branch. Alternatively, a corner of a common portion of the interdigital electrode or a reflector electrode is cut off.
These inventions are disclosed in Japanese Patent Laid-Open Publication Nos.07-263994 and 09-186543.
FIGS. 6A and 6B are a plan view and an enlarged partial view of a conventional SAW device, respectively. The device includes piezoelectric substrate 1, interdigital electrodes 2, reflector electrodes 3, common portion 4 of each interdigital electrode 2, and common portions 5 of each reflector electrode 3. In the conventional interdigital electrodes, interdigitating portions are formed on the same propagation path to allow a surface acoustic wave to propagate in a single direction, thus reducing the conversion loss resulting from different propagating directions. However, if a potential difference between the electrodes adjacent to each other increases during the forming of the electrodes, the resulting electrode has a curved shape in section, which causes energy of the surface acoustic wave to leak out. This leakage increases propagation loss. Although the electrodes having respective corners of the common portions cut off can be prevented from a short circuit between the electrodes, the finger still has the curved shape in section, and therefore, the energy of the surface acoustic wave leaks out, thus increasing the propagation loss increases.